This research is aimed at providing an understanding of the mechanisms used by Schistosoma mansoni to survive and multiply in its intermediate host snail Biomphalaria glabrata. S. mansioni is a causative agent of human schistosomiasis, a serious debilitating disease in human populations in many areas of the tropics. On the premise that the compatibility of S. mansoni with B. glabrata is a highly specific phenomenon, and that the specificity is manifested as a result of events occurring at the interface of the sporocyst with host-snail hemocyte plasma membranes, we aim to provide information on the cellular-chemical basis of specificity. Experiments will be designed to yield information which will support or refute various hypotheses that bear directly on the following issues (a) The mechanisms by which snail hemocytes recognize trematode sporocysts (b) The mechanisms by which snails acquire enhanced resistance, of a more or less specific nature, to trematode sporocysts. (c) The mechanisms by which trematode sporocysts avoid destruction by hemocytes of certain snail strains (d) The mechanisms by which trematode sporocysts interfere with innate resistance of snails Methodologies will involve in vitro culture of schistosome sporocysts larvae together with immunocompetent leukocytes (hemocytes) of host sanils. Both susceptible and resistaint strains of snail will be used as sources of hemocytes. Sensitive labelling of potentially important molecules of host and parasite origin will permit us to evaluate their roles in mediation of compatibility.